The present invention relates to a method and system for liquefaction of a gas stream, more specifically, to a system and method for liquefaction of a natural gas stream in a natural gas liquefaction plant. Systems for cooling, liquefying, and optionally sub-cooling natural gas are well known in the art, such as the single mixed refrigerant (SMR) cycle, propane pre-cooled mixed refrigerant (C3MR) cycle, dual mixed refrigerant (DMR) cycle, C3MR-Nitrogen hybrid (such as the AP-X® process) cycles, nitrogen or methane expander cycle, and cascade cycles. Typically, in such systems, natural gas is cooled, liquefied, and optionally sub-cooled by indirect heat exchange with one or more refrigerants. A variety of refrigerants might be employed, such as mixed refrigerants, pure components, two-phase refrigerants, gas phase refrigerants, etc. Mixed refrigerants (MR), which are a mixture of nitrogen, methane, ethane/ethylene, propane, butanes, and optionally pentanes, have been used in many base-load liquefied natural gas (LNG) plants. The composition of the MR stream is typically selected based on the feed gas composition and operating conditions.
The refrigerant is circulated in a refrigerant circuit that includes one or more heat exchangers and one or more refrigerant compression systems. The refrigerant circuit may be closed-loop or open-loop. Natural gas is cooled, liquefied, and/or sub-cooled by indirect heat exchange against the refrigerants in the heat exchangers.
Each refrigerant compression system includes a compression circuit for compressing and cooling the circulating refrigerant, and a driver assembly to provide the power needed to drive the compressors. The refrigerant compression system is a critical component of the liquefaction system because the refrigerant needs to be compressed to high pressure and cooled prior to expansion in order to produce a cold low pressure refrigerant stream that provides the heat duty necessary to cool, liquefy, and optionally sub-cool the natural gas.
In designing and operating an LNG liquefaction plant the selection of heat exchangers, compressors and related equipment is a significant consideration affecting the cost of constructing and operating the plant. A typical prior art system consists of a two-step process whereby a natural gas feed is precooled in a pre-cooler heat exchanger to sub-ambient temperature and then condensed (liquefied) in a main cryogenic heat exchanger (MCHE).
During the pre-cooling step of a typical prior art system, the natural gas to be liquefied is pre-cooled in the hot side (or end) of a pre-cooling heat exchanger by heat exchange with refrigerant evaporating in the cold side. Evaporated refrigerant is removed from the cold side of the heat exchanger. This evaporated refrigerant is liquefied in the pre-cooling refrigerant circuit. To this end, the refrigerant is compressed in a compressor to an elevated pressure, and the heat of compression and the heat of vaporization are removed in a condenser. The liquid refrigerant is allowed to expand in the expansion device to a lower pressure, and at this pressure the refrigerant is allowed to evaporate in the cold side of the natural gas pre-cooling heat exchanger.
There have been efforts in the prior art directed towards the design of the pre-cooling system in order to achieve greater capacity, efficiency and to reduce cost. The use of multiple pre-coolers in series is one such approach. For example, it is known in the art to use two pre-cooler heat exchangers in series and two parallel MCHE's to increase the production rate of a single liquefaction train while reducing the capital expenditure below that of a system using two smaller parallel trains.
It is also known in the art that two pre-cooler heat exchangers in series, a scrub column and a single MCHE can achieve colder gas feed temperatures and improved liquefaction efficiency.
Another approach is the use of parallel cooling cycles. For example, at least one known system uses two identical pre-cooler heat exchangers in parallel with two parallel compression trains and a single MCHE. The two identical exchangers each handle 50% of the load and are intended to be identical (i.e., identical structure, identical stream inputs, identical refrigeration, and identical stream outputs) to simplify the design and manufacturing of the plant and to provide efficiency of maintenance costs. Each component of the system (compressors, heat exchangers, etc.) are selected from the largest available on the market to reduce the number of components required and to minimize capital and operating costs. The two parallel identical heat exchanger configuration provides the advantages of: (a) increasing capacity of the plant to the maximum possible production rate, achieved by maximizing the size of each exchanger within manufacturing and transportation limits; and (b) increasing the capacity of the plant to some intermediate production rate higher than production achieved by using a single exchanger.
In addition, capital investment savings, shortened manufacturing time ease of operation and maintenance are some of the well-known benefits of using duplicate equipment in parallel. However, providing identical parallel heat exchangers also presents several challenges. For example, each heat exchanger must cool multiple streams having different heat demands. The two exchangers must also be well-balanced during operation to assure equal duties and to avoid so-called manifold effect—i.e., different flows in pipes branching from the main pipe due to the varying distance from the main pipe inlet and, therefore, varying frictional pressure losses. This adds complexity to the operation of the system and introduces inefficiencies due to the compromises which must be made to keep the exchangers balanced.
Another disadvantage of using multiple identical heat exchangers is the need for an increased number of cooling circuits. For example, assuming two parallel identical heat exchangers are used to cool each of three different streams: the gas feed stream; the warm mixed refrigerant (WMR); and cold mixed refrigerant (CMR), six cooling circuits will be required. This adds complexity to the system and makes the addition of a second identical heat exchanger in parallel impractical for many existing systems.
Accordingly, there is a need to develop a process for natural gas liquefaction that allows for combining multiple exchanger designs while shortening the manufacturing time, simplifying process control, minimizing the number of refrigeration circuits, improving efficiency and increasing LNG production. Such arrangement should preferably be suitable for use as a retrofit or for a new design.